Visual Acuity, its Development and Amblyopia

Foveal neural adaptation to optically induced contrast reduction

Contrast processing is suggested to interact with eye growth and myopia development. A novel contrast-reducing myopia control lens design decreases image contrast and was shown to slow myopia progression. Limited insights exist regarding neural visual processing following adaptation to image contrast reduction. This study investigated foveal neural contrast sensitivity in 29 young adults following a 30-minute adaptation to scattering using a Bangerter occlusion foil 0.8, +0.5-diopter defocus, and a clear lens control condition. Neural contrast sensitivity at its peak sensitivity of 6 cycles per degree was assessed before and after adaptation to the lens conditions, employing a unique interferometric system. Pre-adaptation measurements were averaged from six replicates and post-adaptation measurements by the first and last three of six replicates. The change in neural contrast sensitivity was largest for scattering across the first and last three post-adaptation measurements (+0.05 ± 0.01 logCS and +0.04 ± 0.01 logCS, respectively) compared with control and defocus (all +0.03 ± 0.01 logCS). For scattering, the observed increase of neural contrast sensitivity within the first three measurements differed significantly from the pre-adaptation baseline (p = 0.04) and was significantly higher compared with the control condition (p = 0.04). The sensitivity increases in the control and defocus conditions were not significant (all p > 0.05). As the adaptation effect diminished, no significant differences were found from baseline or between the conditions in the last three measurements (all p > 0.05). When post-adaptation neural contrast sensitivities were clustered into 25-second sequences, a significant effect was observed between the conditions, with only a significant relevant effect between control and scattering at 25 seconds (p = 0.04) and no further significant effects (all p > 0.05). The alteration in neural contrast sensitivity at peak sensitivity was most pronounced following adaptation to the scattering condition compared with defocus and control, suggesting that induced scattering might be considered for myopia control.

Foveal avascular zone and macular vessel density after correction for magnification error in unilateral amblyopia using optical coherence tomography angiography

Background

To investigate the area of foveal avascular zone (FAZ) and macular vessel density (VD) after correction for magnification error in unilateral amblyopia using optical coherence tomography angiography (OCTA).

Methods

Participants comprised 15 patients with unilateral amblyopia due to anisometropia with or without strabismus (mean age, 9.8 ± 3.4 years; range, 6–17 years). OCTA images were obtained by using spectral-domain OCT with angiography software. The OCTA scanning protocol used was 3 × 3-mm volume scan centered on the fovea. OCTA images were corrected for magnification errors using individual axial length (AL), and an adjusted 2.3 × 2.3-mm square was derived as a region of interest. The FAZ area and VD in both superficial capillary plexus (SCP) and deep capillary plexus (DCP) layers, foveal minimum thickness (FMT) were assessed using built-in OCTA software and ImageJ software (NIH, Bethesda, MD).

Results

LogMAR in the amblyopic eyes was significantly poorer than that of the fellow eye (p < 0.001). AL was significantly shorter in the amblyopic eye than in the fellow eye (p < 0.001). FAZ area of SCP in amblyopic eyes was significantly smaller than that of fellow eyes (p < 0.001). No significant differences were seen in FAZ area of DCP, VD of SCP, VD of DCP, and FMT between amblyopic and fellow eyes (p = 0.07, 0.43, 0.55, and 0.25, respectively).

Conclusions

Our present study after magnification error correction found smaller FAZ area of SCP in the amblyopic eye compared with the fellow eyes, but there was no significant difference in the macular VD between the amblyopic and fellow eyes.

Updates on managements of pediatric cataract

Purpose

A comprehensive review in congenital cataract management can guide general ophthalmologists in managing such a difficult situation which remains a significant cause of preventable childhood blindness. This review will focus on surgical management, postoperative complications, and intraocular lens (IOL)-related controversies.

Methods

Electrical records of PubMed, Medline, Google Scholar, and Web of Science from January 1980 to August 2017 were explored using a combination of keywords: "Congenital", "Pediatric", "Childhood", "Cataract", "Lens opacity", "Management", "Surgery", "Complication", "Visual rehabilitation”, and "Lensectomy". A total number of 109 articles were selected for the review process.

Results

This review article suggests that lens opacity obscuring the red reflex in preverbal children and visual acuity of less than 20/40 is an absolute indication for lens aspiration. For significant lens opacity that leads to a considerable risk of amblyopia, cataract surgery is recommended at 6 weeks of age for unilateral cataract and between 6 and 8 weeks of age for bilateral cases. The recommended approach in operation is lens aspiration via vitrector and posterior capsulotomy and anterior vitrectomy in children younger than six years, and IOL implantation could be considered in patients older than one year. Most articles suggested hydrophobic foldable acrylic posterior chamber intraocular lens (PCIOL) for pediatrics because of lower postoperative inflammation. Regarding the continuous ocular growth and biometric changes in pediatric patients, under correction of IOL power based on the child's age is an acceptable approach. Considering the effects of early and late postoperative complications on the visual outcome, timely detection, and management are of a pivotal importance. In the end, the main parts of post-operation visual rehabilitation are a refractive correction, treatment of concomitant amblyopia, and bifocal correction for children in school age.

Conclusions

The management of congenital cataracts stands to challenge for most surgeons because of visual development and ocular growth. Children undergoing cataract surgery must be followed lifelong for proper management of early and late postoperative complications. IOL implantation for infants less than 1 year is not recommended, and IOL insertion for children older than 2 years with sufficient capsular support is advised.

Macular retinal and choroidal thickness in unilateral amblyopia using swept-source optical coherence tomography

Background

To investigate macular retinal and choroidal thickness in amblyopic eyes compared to that in fellow and normal eyes using swept-source optical coherence tomography (SS-OCT).

Methods

This study examined 31 patients with hyperopic anisometropic amblyopia (6.9 ± 3.8 years, mean ± standard deviation), 15 patients with strabismic amblyopia without anisometropia (7.9 ± 4.2 years), and 24 age-matched controls (7.8 ± 3.3 years). Retinal and choroidal thickness was measured by 3D scans using SS-OCT. A 6-mm area around the fovea was automatically analyzed using the Early Treatment Diabetic Retinopathy Study map. The thickness from SS-OCT was corrected for magnification error using individual axial length, spherical refraction, cylinder refraction, and corneal radius. Retinal thickness was divided into the macular retinal nerve fiber layer (mRNFL), ganglion cell layer + inner plexiform layer (GCL+IPL), ganglion cell complex (GCC), and the inner limiting membrane to the retinal pigment epithelium (ILM-RPE) thickness. Retinal and choroidal thickness was compared among amblyopic, fellow, and normal eyes.

Results

In both amblyopia groups, there was no significant difference in the mRNFL, GCL+IPL, and GCC thicknesses among the amblyopic, fellow, and control eyes. In the anisometropic amblyopia group, choroidal thickness (subfovea, center 1 mm, nasal and inferior of the inner ring, nasal of the outer ring, and center 6 mm) of amblyopic eyes were significantly greater than that of fellow and normal eyes. In contrast, none of the choroidal thicknesses were significantly different among the investigated eyes in the strabismic amblyopia group.

Conclusions

We found no significant difference in inner retinal thickness in patients with unilateral amblyopia. Although there were significant differences in choroidal thickness with hyperopic anisometropic amblyopia, there was no significant difference for the strabismic amblyopia. The discrepancy in choroidal thickness between the two types of amblyopia may be due to both differences in ocular size and underlying mechanism.

Multiple Roles for Nogo Receptor 1 in Visual System Plasticity

During the developmental critical period for visual plasticity, discordant vision alters the responsiveness of neurons in visual cortex. The subsequent closure of the critical period not only consolidates neural function but also limits recovery of acuity from preceding abnormal visual experience. Despite species-specific differences in circuitry of the visual system, these characteristics are conserved. The nogo-66 receptor 1 (ngr1) is one of only a small number of genes identified thus far that is essential to closing the critical period. Mice lacking a functional ngr1 gene retain developmental visual plasticity as adults and their visual acuity spontaneously improves after prolonged visual deprivation. Experiments employing conditional mouse genetics have revealed that ngr1 restricts plasticity within distinct circuits for ocular dominance and visual acuity. However, the mechanisms by which NgR1 limits plasticity have not been elucidated, in part because the subcellular localization and signal transduction of the protein are only partially understood. Here we explore potential mechanisms for NgR1 function in relation to manipulations that reactivate visual plasticity in adults and propose lines of investigation to address relevant gaps in knowledge.

The assessment of 'vision for development' in severely visually handicapped babies

Vision makes a substantial contribution to the pattern and rate of normal development. Considering infant development in 3 phases, global, integrative and specific the most important attributes to vision of development within each phase are discussed. A method for assessment of the aspects of vision most relevant to each child's current developmental needs is described and a philosophy outlined whereby the findings can be represented as a balanced practical prescription for development. The development of the visual system is often delayed in visually handicapped babies; some reasons are put forward and a concern that many may never achieve their potential maximum visual competence is expressed. A method for evaluating the current stage of visual development in a visually handicapped baby is described in which the findings form the starting point of a programme towards achieving maximum visual competence as young as possible.

Thickness of the retinal nerve fiber layer in patients with anisometropic and strabismic amblyopia

PURPOSE

To compare the retinal nerve fiber layer [RNFL] thickness of the amblyopic eye with that of the normal eye in patients with anisometropic and strabismic amblyopia.

METHODS

Scanning laser polarimetry was performed with the GDx Nerve Fiber Analyzer (NFA), a scanning laser ophthalmoscope that measures the thickness of the peripapillary RNFL quantitatively and objectively, in vivo, based on the birefringence of the RNFL. Eighteen anisometropic, two strabismic and four combined amblyopic (both anisometropia and strabismus) patients were enrolled in the study. There were 9 male and 15 female patients, age range 7-66 years. Using Student's t-test, 14 GDx parameters in the amblyopic eyes and sound eyes were compared.

RESULTS

The GDx parameters showed no significant differences between the two groups [p > 0.5]. The superior average and inferior average values were 79.50 microm and 80.41 microm, respectively, in the amblyopic group, and 80.75 microm and 82.75 microm, respectively, in the healthy eyes. The average thickness did not differ between amblyopic and sound eyes (65.50 microm and 66.45 microm, respectively).

CONCLUSION

Assessment of RNFL thickness by means of scanning laser polarimetry revealed no difference between the two eyes in patients with unilateral amblyopia.

Photoreceptor function in unilateral amblyopia

We investigated whether photoreceptor function in amblyopic eyes differed from that in non-amblyopic eyes. Photoreceptor function was assessed with the optical Stiles-Crawford effect (SCE), psychophysical SCE, and foveal visual pigment density in both eyes of ten unilateral amblyopic subjects. Optical SCE and density measurements were carried out with a custom-built scanning laser ophthalmoscope (SLO). Amblyopic and normal eyes did not differ in Stiles-Crawford effect, nor in foveal visual pigment density. Contrary to suggestions in the literature, we found no indication of retinal dysfunction at the level of the cone photoreceptors in amblyopic eyes.

Eye alignment and cortical binocularity in strabismic kittens: a comparison between tenotomy and recession

Interocular alignment was assessed by corneal light reflex photography in 15 normal and 26 strabismic kittens. Strabismus was induced at 3-4 weeks of age by severing one extraocular muscle (tenotomy), by cutting and reinserting the muscle at another position on the ocular globe (recession), or by combining recession of the medial rectus muscle with resection of the lateral rectus muscle of the same eye. Nineteen strabismic and five normal kittens were followed longitudinally from 12 days to about 6 months of age. Three out of six longitudinally followed tenotomized cats and six out of the 13 recessed cats conserved their postoperative ocular deviation throughout the testing period ("large-angle strabismics"). Three tenotomized and seven recessed cats showed a transient deviation for 1-2 weeks after surgery, after which the interocular deviation diminished to values found in normal cats ("microstrabismic" cats). Both recessed-resected cats showed a transient interocular deviation. In spite of their different developmental histories, all cats showed a clear breakdown of binocularity in area 17. Large-angle strabismics showed a dominance of the non-operated eye, while in microstrabismic cats, both eyes were equally effective in driving cortical cells. It thus appears that a transient strabismus is sufficient to produce a reduction of binocularity in area 17.

Excitatory amino acid receptors on sustained retinal ganglion cells in the kitten during the critical period of development

Effects of iontophoretically applied excitatory amino acid analogues, kainate, quisqualate and N-methyl-D-aspartate (NMDA) and their receptor antagonists on the sustained class of retinal ganglion cells were studied in the optically intact eye of pentobarbitone-anaesthetized kittens (7-9 weeks of age). These results were compared with the effects obtained in adult cats. All 3 excitatory amino acid agonists had excitatory actions on the majority of On- and Off-sustained ganglion cells in the kitten but at higher current levels than those required for adult cells, suggesting all 3 types of receptors of weaker sensitivity are present on the kitten cells. Whilst the relative potency of kainate, quisqualate and NMDA was 15:3:1 in the adult cells, it was 5:2:1 in the kitten cells. As for other neurones in the CNS, an increase in the potency of kainate receptors and a decrease in that of NMDA receptors appear, therefore, to characterize the postnatal development of the excitatory amino acid receptors on the retinal ganglion cells. In accordance with the agonist results, a broadband receptor antagonist, kynurenate, powerfully antagonised responses of kitten cells as well as those of adult cells. The pure NMDA receptor antagonist, 3((+-)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), however, only suppressed spontaneous firing of kitten cells. Furthermore, in kitten cells, the visually-driven firing was depressed while the level of firing was raised by these excitatory amino acid analogous, and a long period of inhibition of firing followed the agonist-induced excitation.(ABSTRACT TRUNCATED AT 250 WORDS)

The pattern electroretinogram

Physiological experiments and the exploitation of clinical conditions have provided compelling evidence that retinal ganglion cells and other inner retinal structures generate the pattern ERG (PERG). As an increasing number of clinical reports have been published some contradictory findings have been reported. These may be ascribed to variation in recording and measuring techniques. The PERG consists of two major portions, the early positive and the following negative component which can be investigated separately if the stimulus conditions allow isolated (or "transient") responses to be recorded. Care has to be taken in positioning the reference electrode, maintaining accurate refraction, and the influence of pupil size must be considered. Furthermore the PERG is contaminated by a luminance component which may be generated in the outer retina. The size of this increases with low spatial frequency (large check-sizes) and high mean luminance. The PERG permits the examination of an additional level of the retina and helps the understanding of pathophysiology of various eye diseases, and is of clinical importance in routine diagnosis and assessment. In glaucoma the PERG amplitude is often reduced before it is possible to detect a scotoma and it is therefore an important prognostic indicator in patients with ocular hypertension. In diabetic retinopathy, retinal ischaemia sufficient to lead to the pre-proliferative state can be demonstrated. The PERG also has a major clinical role in examining localised retinal pathology. If combined with VECP recording, it greatly extends the interpretations possible, since not only can damage to the optic nerve be detected by both tests, but the normal PERG in the presence of an abnormal PVECP implies that the losses are confined to the central pathway.

Alteration of the visual evoked potential by macular holes: comparison with optic neuritis

Nine patients with maculopathy (macular holes, macular cysts, and lamellar holes) and ten patients with optic neuritis were examined in order to determine changes in the visual evoked potential (VEP) in response to pattern-reversal stimulation. Eyes with lamellar holes had normal P100 latency, but eyes with macular cysts and macular holes had prolonged P100 latency. Eyes with optic neuritis exhibited greater prolongation of the P100 latency than eyes with macular holes. In contrast, eyes with macular holes had a greater reduction in the steady-state VEP amplitude than eyes with optic neuritis. The prolonged latency occurring in maculopathy may be due to a peculiar amplitude summation noted with half-field VEP, rather than to a true conduction delay like that seen in eyes with optic neuritis. The amplitude slope, which is usually positive in normal controls, was negative for 85.7% of eyes with macular holes and 69.2% of eyes with optic neuritis. The negative amplitude slope may represent a subtle defect in retinal ganglion X cells. Eyes with significantly lower values for four or more of the nine central test points on quantitative automated perimetry had negative amplitude slopes and prolonged P100 latency.

Postnatal development of GABA- and glycine-mediated inhibition of feline retinal ganglion cells in the area centralis

Intraretinal iontophoresis in the optically intact eye of adult cats (18-22 weeks of age) and kittens (7-9 weeks of age) under pentobarbitone anaesthesia was performed. Studies were concentrated on retinal ganglion cells of the sustained (X) type in the area centralis under photopic conditions. In both the adult and kitten, gamma-aminobutyric acid (GABA) and muscimol inhibited the visually induced excitation, and bicuculline blocked the visually induced inhibition of on-cells. On the other hand, glycine inhibited the excitation and strychnine blocked the inhibition of off-cells. However, a greater current of GABA (muscimol) and glycine was required to produce total inhibition in kitten's on- and off-cells respectively when compared with the adult's. Furthermore, a smaller current of bicuculline and strychnine was needed to abolish the visually induced inhibition of kitten on- and off-cells respectively when compared with the adult's. In the adult, GABA and glycine did not affect the responses of off- and on-cells respectively, but in the kitten GABA inhibited off-cells and glycine inhibited on-cells to some extent. In neither the adult nor the kitten did bicuculline have any effect upon off-cells or strychnine any effect upon on-cells. Thus, the sustained on- and off-cells in the kitten area centralis exhibit: a reduced selectivity to inhibitory transmitters; a reduced sensitivity to exogenously applied inhibitory transmitter agonists; but a greater sensitivity to inhibitory transmitter antagonists, in comparison with the sustained on- and off-cells in the adult area centralis. The observed differences between the kitten and adult cat in transmitter actions on retinal ganglion cells appear to be analogous to those found in the postnatal development of functional synapses at the neuromuscular junction and sympathetic ganglia.

The cat as a model for visual deprivation

Both behavioural and neurophysiological changes can be observed in cats that have experienced interference with their normal visual environment. This visual deprivation may result from alterations to the path of light forming the normal image on the retina, and includes changes that cause the image to fall on an inappropriate part of the retina so that normal binocular interactions are affected. While some neurophysiological changes can be observed at the level of the lateral geniculate nucleus they become more prominent as information reaches the visual cortex, where cells commonly receive neural excitation from both eyes and require the information to come from corresponding parts of the two retinas and that the stimulus should have appropriate orientation and direction of movement. Many of the observations of deprivation in animals have clear parallels in the human environment.

Is the retina sensitive to the effects of prolonged blur?

Two preparations were used to study the developmental effects of prolonged blurring of retinal images on the acuities of retinal ganglion cells. Five kittens were raised from three weeks to six months of age with daily administration of atropine to one eye. Another two kittens were raised from three weeks to 16 weeks with a contact lens of high refractive power fitted to one eye. Behavioural estimates of the visual acuity were made for two animals from each group. Animals of both groups demonstrated an amblyopia in the experimental eye: visual acuity varied from 1.8 to 2.5 cycles per degree compared with 6.0 to 7.5 cycles per degree when using the normal eye. The spatial resolving properties were measured for retinal ganglion cells within the amblyopic eyes of two lens-reared cats and three atropinized cats. Brisk-sustained (X) cells were recorded from along the naso-temporal division. The acuities of ganglion cells from the lens-reared cats were indistinguishable from those from normal cats at comparable eccentricities. However, for the cats raised with atropine administration, sub-normal acuities were determined for retinal ganglion cells from all regions that were studied in the experimental eye. We conclude that blur of retinal images produced by external means has no effect on the resolving power of retinal ganglion cells. The lowered ganglion cell acuities encountered with the atropinized cats must be attributable to a secondary effect of the atropine administration. Organic changes in the retinal blood vessel pattern support this contention.

A developmental approach to sensory disabilities in early childhood

Rehabilitation for young children with hearing and visual disabilities is considered from a developmental angle. The effect of these sensory disabilities on parents; the constraints on development imposed by each disability; and medical and surgical treatment and the use of aids in young children are discussed.

Developmental physiological optics and visual acuity: a brief review

The development of optical and neural factors affecting visual acuity is reviewed with the aim of determining the age at which the relationship between optical and neural factors become mature. Delayed development of extrastriate cortical and indirect visual pathways may account for differences in acuity assessed by preferential looking and pattern reversal VEPs.

Vision testing of young children in the age range 18 months to 4 1/2 years

Four hundred and twenty-five normal children in the age range 18 months to 4 1/2 years have been studied to determine the percentage of children according to age who can be expected to cooperate in a letter-matching vision test using a key card; to see whether the use of plastic letters to replace the key card lowers the age at which a letter-matching test can be achieved, and to show that children in this age range can do a vision test at 20ft (6m). Acceptance of occlusion using a patch occluder was also examined. The study shows that children effectively begin to cooperate in letter-matching vision tests using a key card from 33 months of age. Replacement of the key card with plastic letters lowers the age to 30 months and thereafter there is a gain of some 25% of children up to 39 months. All children were successfully tested at 20ft (6m). The most difficult age for occlusion was in the age group around the second birthday.

Latency of visually evoked saccadic eye movements. II. Temporal properties of the facilitation mechanism

The validness of a model describing the relation between mean saccadic latency and stimulus asynchrony based on facilitation instead of suppression was tested experimentally. As a result, suppression of signals generated by the onset of a peripheral stimulus due to fixation of another target, giving rise to an increase of mean saccadic latency, does not seem very likely. The influence of the intensity of the fixation target on the latency of visually evoked saccades was studied. According to the facilitation model, the offset of the fixation target induces after an afferent delay, a transition of the state of the facilitation mechanism from the unfacilitated condition into a mode of maximal facilitation. The time-period during which this change is accomplished is called Facilitation-Rise-Time (FRT). An interpretation within the context of the facilitation model of gap-overlap latency data for different values of the intensity of the fixation stimulus suggests, in combination with computer-computations of the model, that lowering of this intensity causes an increase in FRT. The results in normal subjects of step stimulus experiments with a dim fixation point substantiate the hypothesis of a facilitation mechanism, which is triggerable not only by an external signal such as the offset of the fixation point, but also by some internal stimulus independent signal. Moreover, data for tracking by an amblyopic eye seem to support this conclusion. The findings of increased saccadic latencies in amblyopic and Optic Neuritis (ON) eyes suggest a slowing of processing of visual information in the sensory pathways from the central retina, subsequently utilized by the oculomotor system in the generation of saccades.(ABSTRACT TRUNCATED AT 250 WORDS)

Pupillary defects in amblyopia

We examined the pupils of 55 amblyopic subjects to determine whether a pupillary defect could be detected in the amblyopic eye. We found relative afferent pupillary defects in 45 of the subjects; these defects were equal to or larger than 0.3 log unit (that is, easily visible) in 29 subjects. The pupillary defect was always in the amblyopic eye, but it could not be correlated with the severity or the cause of the amblyopia, with visual-evoked potential abnormalities, or with color vision defects.

Monocular rotary nystagmus

A patient with transient monocular rotary-vertical nystagmus demonstrated decreased gain of vertical pursuit and normal vestibulo-ocular reflex on electrooculogram. A supranuclear brain stem lesion, resulting in lack of monocular inhibition of oculomotor neurons, is postulated on the basis of these findings.

Two spatio-temporal filters in human vision. 1. Temporal and spatial frequency response characteristics

We have studied visual detection of a circular target moving across a spatially and/or temporally modulated background. Illumination, It, for threshold detection of the target has been measured as a function of background modulation frequency and changes in It associated with background modulation provide a means of determining the frequency response characteristics of visual channels. Temporal frequency responses obtained with temporally modulated, spatially uniform backgrounds have pass-band characteristics and the temporal frequency for peak response increases with increase in mean background illumination. These temporal frequency responses resemble those of the de Lange (1954) filter, but the latter incorporates the incremental thresholds for steady backgrounds. The amplitude of this temporal response saturates at low (approximately 40%) background modulation, decreases to zero as the target velocity falls to zero, and is maximum for a circular target of diameter 2 degrees. The spatial characteristics of this temporal filter were measured with a background field consisting of alternate steady and flickering bars. The resulting spatial frequency curve peaks at 1 cycle deg-1 for all background illuminations and is independent of the background grating orientation. This spatial response differs significantly from the IMG spatial functions observed with a background grating (Barbur and Ruddock, 1980). The spatial and temporal responses reviewed above exhibit similar parametric variations and we therefore associate them with a single spatio-temporal filter, ST2. A second temporal response, with low-pass frequency characteristics, was observed with a background field consisting of two matched gratings, presented in spatial and temporal antiphase. This response has parametric properties similar to those of the IMG spatial response described previously by Barbur and Ruddock (1980), thus we associated the two sets of data with a single spatio-temporal filter, ST1. We show that the ST2 responses can be obtained by combining ST1 responses, and we present a network incorporating the two filters. We review other psychophysical studies which imply the activity of two spatio-temporal filters with properties of the kind revealed in our studies. We argue that filter ST1 has properties equivalent to those of X-type and filter ST2 has properties equivalent to those of Y-type electrophysiological mechanisms.

Two spatio-temporal filters in human vision. 2. Selective modification in amblyopia, albinism, and hemianopia

We have used the psychophysical methods described in the first paper of this series (Holliday and Ruddock, 1983) to determine selected spatial and temporal response characteristics of the ST1 and ST2 filters for subjects suffering visual defects. Data are given for 19 amblyopes, an albino and a hemianope, and comparison data are also given for a number of subjects with normal vision. The ST1 spatial responses for both the "normal" and "amblyopic" eyes of 12 convergent strabismic amblyopes are displaced to low spatial frequencies compared to the normal curve, which implies that there is a loss of fine spatial tuning. In all but one subject, the curve for the "amblyopic" eye peaks at a spatial frequency lower than that for the "normal" eye, thus the former deviates further from the normal pattern than the latter. The ST1 spatial responses of 6 refractive amblyopes are also displaced to the low frequency side of the normal curve, although on average the shift is smaller than in the case of the strabismic amblyopes. For each subject, the response curve of the "amblyopic" eye peaks at a lower spatial frequency than does that for the "normal" eye. ST1 spatial responses were measured for targets located up to 30 degrees off-axis along the horizontal meridian and sample data are given for one strabismic and one refractive amblyope and for two normal subjects. It is concluded from these data that the changes in the spatial responses associated with amblyopia do not simply reflect eccentric fixation of the target. The ST2 spatial response was measured for the "normal" and "amblyopic" eyes of 9 amblyopes (7 strabismic and 2 refractive). There is no significant difference between the average amblyopic response and that of normal subjects, and only in one case does the response for an "amblyopic" eye peak at a frequency lower than the peak frequency for normal vision. The ST2 temporal response for 9 amblyopes shows no systematic deviations from the normal response. For the albino, both the ST1 and ST2 spatial responses peak at around 0.3 cycles deg-1, and both curves are displaced considerably to the low spatial frequency side of the normal ST2 spatial response. The albino's ST2 temporal response is essentially normal. Measurements for the hemianope's "blind" hemifield under conditions appropriate to the isolation of the ST1 and ST2 spatial responses reveal no tuning curves. The ST2 temporal response for the "blind" hemifield, however, is of large amplitude, with a peak at 2 Hz, well below the normal frequency response peak. It is argued that the loss of fine spatial tuning which occurs in the ST1, but not the ST2, spatial responses of the amblyopes is consistent with the sequential organisation of these two filter classes proposed by Holliday and Ruddock (1983). Further, for the only two subjects whose ST2 spatial response curves are displaced to abnormally low frequencies (the albino and a strabismic amblyope) the ST1 spatial response is shifted to low spatial frequencies compared to the normal ST2 curve...

Indications for prescribing spectacles

Indications for spectacle correction are reviewed in the context of symptoms, pathophysiology, and prognostic efficacy. Criteria include improving acuity, restoring comfortable vision, enhancing visual efficiency, preventing progression, and assorted miscellaneous goals. Alternatives to spectacles and side effects are analyzed. Specific guidelines for individualized management are evaluated in terms of current research.